3974
The Journal of Experimental Biology 209, 3974-3983
Published by The Company of Biologists 2006
doi:10.1242/jeb.02482
Regulation of stroke pattern and swim speed across a range of current velocities:
diving by common eiders wintering in polynyas in the Canadian Arctic
Joel P. Heath1,*, H. Grant Gilchrist2 and Ronald C. Ydenberg1
1
Centre for Wildlife Ecology / Behavioural Ecology Research Group, Department of Biological Sciences, Simon
Fraser University, Burnaby, British Columbia, V5A 1S6, Canada and 2National Wildlife Research Centre, Canadian
Wildlife Service, 1125 Colonel by Drive, Raven Road, Carleton University, Ottawa, Ontario, K1A 0H3, Canada
*Author for correspondence (e-mail: jpheath@sfu.ca)
Accepted 9 August 2006
Summary
Swim speed during diving has important energetic
nearly simultaneous strokes of wings and feet, and swim
consequences. Not only do costs increase as drag rises nonspeed relative to the moving water, were maintained
linearly with increasing speed, but speed also affects travel
within a narrow range (2.28±0.23·Hz; 1.25±0.14·m·s–1,
time to foraging patches and therefore time and energy
respectively). This close regulation of swim speed over a
budgets over the entire dive cycle. However, diving
range in current speed of 1.0·m·s–1 might correspond to
behaviour has rarely been considered in relation to
efficient muscle contraction rates, and probably reduces
current velocity. Strong tidal currents around the Belcher
work rates by avoiding rapidly increasing drag at greater
Islands, Nunavut, Canada, produce polynyas, persistent
speeds; however, it also increases travel time to benthic
areas of open water in the sea ice which are important
foraging patches. Despite regulation of average swim
habitats for wildlife wintering in Hudson Bay. Some
speed, high instantaneous speeds during oscillatory
populations of common eiders Somateria mollissima
stroking can increase dive costs due to drag. While most
sedentaria remain in polynyas through the winter where
diving birds have been considered either foot or wing
they dive to forage on benthic invertebrates. Strong tidal
propelled, eider ducks used both wing and foot propulsion
currents keep polynyas from freezing, but current velocity
during descent. Our observations indicate that the power
can exceed 1.5·m·s–1 and could influence time and energy
phase of foot strokes coincides with the transition between
costs of diving and foraging. Polynyas therefore provide
upstroke and downstroke of the wings, when drag is
naturally occurring flume tanks allowing investigation of
greatest. Coordinated timing between foot and wing
diving strategies of free ranging birds in relation to
propulsion could therefore serve to maintain a steadier
current velocity. We used a custom designed sub-sea ice
speed during descent and decrease the costs of diving.
camera to non-invasively investigate over 150 dives to a
Despite tight regulation of stroke and swim speed patterns,
depth of 11.3·m by a population of approximately 100
descent duration and total number of foot and wing
common eiders at Ulutsatuq polynya during February
strokes during descent increase non-linearly with
and March of 2002 and 2003. Current speed during
increasing current velocity, suggesting an increase in
recorded dives ranged from 0 to 1·m·s–1. As currents
energetic costs of diving.
increased, vertical descent speed of eiders decreased, while
descent duration and the number of wing strokes and foot
Key words: current, diving, swimming, wing stroke, glide,
biomechanics, locomotion, drag, foot propulsion, underwater video.
strokes during descent to the bottom increased. However,
Introduction
Buoyancy, drag and inertia influence the energy budgets of
air breathing animals that dive to forage. As depth increases,
buoyancy decreases while drag increases to become the
primary energetic cost of diving (Lovvorn, 2001; Lovvorn et
al., 2004). Drag also increases nonlinearly with speed
(Lovvorn, 2001), and recent work using accelerometers on
diving animals has shown that swim speed is often maintained
within a narrow range that likely minimizes energetic costs of
drag (Watanuki et al., 2003; Lovvorn et al., 2004). Propulsion
by wings, flippers or feet during diving includes power and
recovery phases of muscle contraction, and limb orientation
and instantaneous velocity and drag can vary widely
throughout stroke cycles. Additionally, as depth increases
during descent, less surge during the upstroke is required to
counter buoyancy, and downstrokes can provide primary thrust
to maintain speed of descent (Watanuki et al., 2003; Lovvorn
et al., 2004). Different stroke patterns and intermittent gliding
THE JOURNAL OF EXPERIMENTAL BIOLOGY
Diving in currents 3975
can therefore be important in regulating swim speed and
minimizing the energetic costs of diving (Williams et al., 2000;
Watanuki et al., 2003).
Costs of locomotion and the importance of swim speed and
travel time to foraging patches have received considerable
attention in relation to diving depth. However, there is almost
no information concerning how currents affect the behaviour
and energetics of free ranging divers, and how animals regulate
their locomotion across a range of current speeds. Given the
substantially increased effects of drag with speed and
expectation that animals should travel at an average speed that
reduces expensive drag costs [(see Lovvorn et al., 1999;
Pennycuick, 1997) for flight], strong current velocities could
have a considerable influence on locomotion and time and
energy budgets of diving animals.
As drag increases with increasing speed, managing energy
costs of diving entails maintenance of a steady instantaneous
velocity during locomotion, in addition to regulation of average
swim speed. This is because strong drag-induced deceleration
during the recovery phase requires strong acceleration during
the power phase to maintain average velocity (Lovvorn et al.,
1999; Lovvorn and Liggins, 2002). Maintaining a steady
instantaneous velocity could be difficult when diving in
currents. Wing propelled divers can produce thrust during both
the upstroke and down stroke. Greater drag on the wings during
the active part of the upstroke will substantially influence the
animals’ ability to maintain constant velocity for a given work
per stroke and stroke duration (Lovvorn and Liggins, 2002;
Lovvorn et al., 2004). In foot propulsion, little or negative
thrust occurs during the recovery phase, and maintaining
average descent speed requires increased instantaneous
velocity during the power phase, which substantially increases
drag (Lovvorn, 2001; Lovvorn and Liggins, 2002). Preliminary
observations indicate free ranging eiders use both wing and
foot propulsion ubiquitously throughout descent. Co-ordination
between wing and foot propulsion could be particularly
important in overcoming deceleration and maintaining a
steadier speed through stroke cycles.
Energetically efficient diving strategies must also incorporate
efficient muscle contraction rates, which are expected to involve
constant work per stroke (Lovvorn et al., 1999; Kovacs and
Meyers, 2000). Therefore, efficient regulation of average swim
speed can require the maintenance of a constant contraction rate,
stroke duration, and work per stroke, all while altering stroke
frequency and/or gliding between strokes. This has been
observed over increasing depths for both birds and mammals
(Williams et al., 2000; Watanuki et al., 2003; Lovvorn et al.,
2004; Watanuki et al., 2005). Regulation of swim speed through
altering stroke frequency and/or gliding could be particularly
important under increasing currents, as an increase in swim
speed relative to the fluid would be required just to maintain
position in the water column. Given strong non-linear increases
in drag with speed, maximizing energetic economy across
increasing currents likely entails regulating swim speed within a
narrow range that reduces drag costs. Recent work indicates that
there is not a distinct breakpoint of increasing drag with
increasing speed (Lovvorn et al., 2001); however, birds appear
to regulate speed to avoid rapidly increasing drag at higher
speeds (Lovvorn et al., 2004). Accelerometer data also indicate
that efficiency of muscle contraction is an important component
determining efficient work against drag (Lovvorn et al., 2004;
Watanuki et al., 2005). Regardless of the mechanism, as currents
increase, a diver could regulate effective swim speed (relative to
the water) by maintaining stroke duration, work per stroke and
stroke frequency. This would incur a reduced vertical descent
speed, and therefore an increased number of wing strokes and
time required to reach benthic foraging patches. Despite these
particular details, the component of dive costs due to drag is
expected to increase steeply and non-linearly with current
velocity.
Among the Belcher Islands of south-east Hudson Bay, strong
tidal currents pass between islands; in winter, these currents
maintain persistent open water areas called ‘polynyas’. These
polynyas are important winter habitat for a variety of marine
mammals and birds, including common eiders Somateria
mollissima sedentaria, which dive to the sea floor to forage on
benthic invertebrates (Gilchrist and Robertson, 2000). Current
speed in these polynyas vary predictably from 0·m·s–1 at slack
tide, to in excess of 1.5·m·s–1, providing a naturally occurring
flume tank over the tidal cycle. Preliminary observations
showed that common eiders stop foraging and rest on the ice
edge at peak currents, leading us to question how tidal currents
influence their diving and foraging behaviour and affect their
ability to balance energy budgets during the winter in the arctic.
Data obtained from recent advances in animal-borne devices
have helped elucidate diving behaviour of birds and mammals
at sea. However, logger attachment is potentially invasive and
may have negative effects on hydrodynamics of birds (RopertCoudert and Wilson, 2005). Further, both field and captive dive
tank studies have typically been restricted to a small number
of individuals, and individual variation in dive parameters is
often high (e.g. Halsey et al., 2003). In the present study, the
clear arctic water and restricted area of the open water habitat
allowed us to deploy a video camera beneath the ice so that we
could to record complete dives of approximately 100 wintering
common eiders as they descended to forage at a constant depth
(11.3·m). Here we describe the diving behaviour of common
eiders in relation to variation in tidal current velocity ranging
from 0–1.0·m·s–1. We investigate the prediction that eiders
respond to increasing costs of drag as current speed increases
by maintaining relatively constant swim speed and stroke
patterns, at the cost of increasing travel time to the sea floor.
These varying time and energy costs can have strong influence
on foraging patterns over the dive cycle (Houston and Carbone,
1992; Thompson et al., 1993; Boyd, 1997) and could be
particularly important for eiders attempting to balance their
energy budgets in the arctic during winter.
Materials and methods
In collaboration with Inuit from the community of
Sanikiluaq, Nunavut, we studied the diving and foraging
THE JOURNAL OF EXPERIMENTAL BIOLOGY
3976 J. P. Heath, H. G. Gilchrist and R. C. Ydenberg
Sea ice edge
Dive angle
(0−20° in Fig. 5)
Current (0−1.0 m s–1)
)
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Quantifying tidal currents
We quantified tidal current velocities using a Nortek
Aquadopp 3D current meter (Vangkronken, Norway) with a
directional fin deployed in the middle of the water column
(~5.5·m from the surface) on a mooring line anchored to both
the bottom and the sea ice within several meters of the polynya
edge. This device uses three acoustic beams to measure
Doppler velocity to an accuracy of 1% of measured value ±
0.5·cm·s–1 sampled at 23·Hz and set to average velocity over
10·min intervals throughout the duration of the study.
Synchronizing Aquadopp and video time therefore allowed us
to quantify an average current velocity associated with each
eider’s dive.
Calculation of effective swim velocity
We used vector trigonometry to calculate effective swim
speed (m·s–1) relative to the water, based on known vectors of
horizontal current velocity (current) and descent velocity
(descent). Ducks always dived directly into the current and
ended upstream of their departing point. We estimated an
average dive angle of 10° given a known depth of 11.3·m and
an average difference of about 2·m horizontal distance
upstream from where the birds departed the surface and arrived
Ca
Video analysis
From digital videos we recorded the durations of descent,
frequency and cumulative number of wing stroke and foot
stroke cycles during each descent, and the timing of wing
stroke and foot stroke stages. Departure from the surface was
quantified as the frame in which the bill of the bird broke the
surface of the water. The time at which the bird reached the
bottom was determined by a change in the body axis to a
horizontal direction, which was also accompanied by cessation
of wing flaps (only feet and not wings were used while the birds
were foraging at the bottom). The video recorded at 30 frames
per second (f.p.s.), and so maximum accuracy of calculated
durations were 1/30th of a second. Wing flaps were counted by
recording the point at which the wings reached a fully closed
position within the wing stroke cycle. Foot strokes were
counted when the leg reached full extension following the
power phase.
To evaluate if mid-water current velocity was
representative of the water column, we used a Nortek
Aquadopp current profiler mounted below the sea ice near the
edge of the polynya, within 5·m of where eiders were diving.
This instrument allowed quantification of average current
velocity within 0.5·m depth categories across the water
column, at 10·min intervals (with the exception of current
velocities within 0.5·m of the surface and bottom boundaries,
which are not accurately quantified by this instrument).
As data from this instrument indicated that, across the tidal
cycle, there was extremely little variation in current velocity
across depth categories (mean coefficient of variation across
24·h on March 05, 2003 was 0.003±0.002·m·s–1), only data
from the mid-water deployment Aquadopp was used in dive
analyses.
Depth (11.3 m)
behaviour of Common Eiders Somateria mollissima sedentaria
L. wintering at Ulutsa tuk polynya during February and March
of 2002 and 2003. We observed complete dives by deploying
a Sony DCR-TRV730 camera in a modified underwater casing
through a hole chopped in the ice at the edge of the polynya.
At this location eider ducks dived 11.3·m to forage on benthic
invertebrates on the sea floor. We designed a camera support
apparatus that allowed full three-dimensional camera
movement from the surface. A pole mounted in a support frame
allowed us to raise and lower the camera, and rotate it through
360°. The camera casing was mounted on a spring-loaded
platform at the end of the pole, which was connected to a pulley
at the surface. This allowed us to tilt the camera through 180°,
from the surface to the sea floor. Cables extended from the
camera casing to a power source, camera controller, and video
monitor on the surface, and allowed us to film complete dives
of eiders. 157 dives were recorded by opportunistically
following eiders departing the surface, which likely represents
an essentially random sample of the eiders wintering at
Ulutsatuk polynya (approx. 100 individuals). Observations
were conducted during the day, throughout the tidal cycle, as
long as eiders were still diving. Although tides were
semidiurnal, behavioural observations over 24·h periods
indicated that diving at night was extremely rare.
Upstream displacement (~2 m)
Bottom
Fig.·1. A schematic illustrating the measured descent velocity of an
eider diving into currents at the edge of a polynya. Eiders always dived
into currents and ended upstream of their surface departure point. This
schematic illustrates the dive angle and vectors used to calculate
effective swim velocity, relative to the moving fluid, as described in
the text.
THE JOURNAL OF EXPERIMENTAL BIOLOGY
Diving in currents 3977
on the bottom (see Fig.·1). Therefore, we calculated swim
speed relative to the water (swim) as:
Swim =
冪 Descent2 + Current2 – [2⫻Descent⫻Current⫻cos(80)]·.
(1)
For sensitivity analysis, we also present results based on dive
angles ranging from 0° to 20° (i.e. 70° to 90° relative to
horizontal/current direction).
Statistics
Regression analysis was used to determine relationships
between the durations of each diving parameter and current
velocity. Quadratic regression was used when it explained
more of the variation in diving parameters, which was expected
as the effects of drag are known to increase non-linearly with
speed (Lovvorn et al., 1991). JPM 5.0.1.2 (SAS Institute,
Cary, NC, USA) was used for all statistical analysis. Values
presented in the text are mean ± the standard deviation (s.d.).
Results
Wing propulsion
Our observations of free ranging eiders diving to 11.3·m
indicated that both wings and feet were used ubiquitously
during descent. An average wing stroke frequency of 2.28·Hz
(which did not vary with current speed; see below) corresponds
to a stroke cycle duration of 0.439·s, or 13 frames of video at
30·f.p.s. Fig.·2 illustrates, from four perspectives (rows; based
on dives that differed in their position relative to the camera),
the various stages of the stoke cycle in 13 frames at intervals
of 1/30th of a second.
Wing strokes were broken into three phases, the upstroke,
transition and downstroke, based on the orientation of the
leading and trailing edges of the wing. The upstroke was
defined as movement of the wings from completely closed, and
while the leading edge was being raised, without a change in
the angle of attack (Fig.·2, frames 1–6). During the upstroke,
the wings were oriented at a shallow angle of attack relative to
the direction of travel, where water could presumably flow
easily across the wing. This likely lowered drag at this stage.
While this stage of the wing cycle may provide some surge
[downwards momentum (see Watanuki et al., 2003)], the lack
of bubbles generated by vortex shedding of the wing boundary
layer suggests that this was weak. The benign angle of attack
in the direction of travel suggests that forward momentum in
this stage may be primarily through passive gliding. Further,
the head tended to be pointed downwards at the steepest angle,
almost orthogonal to the body axis, during this stage of the
wing stroke cycle (Fig.·2, frame 5).
The transition phase was considered to have begun once the
leading edge of the wing began to travel downwards (Fig.·2,
frame 7). During the transition phase between the upstroke and
downstroke, the trailing edge of the wings were still being
raised and the wings were oriented at a high angle of attack so
that the broad side of the wings were facing the direction of
travel (Fig.·2, frames 7–10). This indicates there was probably
significant drag occurring during this stage, as suggested by
Stettenheim (Stettenheim, 1959) (see also Lovvorn et al.,
2004); however, downwards surge to counter buoyancy could
also be important in this stage (Watanuki et al., 2003; Lovvorn
et al., 2004). Numerous bubbles could be seen shedding off the
tips of the secondary wing feathers in this stage (Fig.·2, frames
8–9, particularly visible in the bottom two rows). During the
transition phase, the head and neck orientation changed from
parallel to its steepest upwards angle with respect to the body
axis (Fig.·2, frame 10).
During the downstroke, both the leading and trailing edge of
the wing were lowered, quickly closing the wings, during
which the majority of propulsion appears to have occurred
(Fig.·2, frames 11–13). The change in the movement of the
trailing edge of the wing from being raised to being lowered is
most obvious in the second row of Fig.·2, between frames 10
and 11, as the tips of the primary feathers change from being
bent slightly backwards to being bent forwards. Although the
steep angle of attack during the transition phase may have
entailed considerable drag, this steep angle probably allowed a
greater surface area of the wings to contact the water and
facilitate propulsion in the downstroke. During the downstroke,
the wings were oriented at a low angle of attack, with the broad
side of the wings orthogonal to the direction of travel, which
presumably reduced drag, increasing forward momentum.
Vortex shedding across the tips of the primary feathers also
appeared to occur during the final stage of the downstroke, in
frames 12 and 13. Additionally, the neck tended to be fully
extended parallel with the body axis throughout frames 12 and
13. Overall, neck and body orientation undulated across the
wing stroke cycle, which left a distinct saw tooth pattern of
bubbles in the water column.
Foot propulsion
Foot strokes were also divided into three categories, thrust,
retract and glide stages. Through the wing stroke cycle, the feet
were most often in the glide phase, which was defined as the
legs being fully extended, parallel with the body and with both
feet tucked under the tail. Most often in this stage, the toes were
closed, which could further minimize drag across the webbing.
Foot retraction, or the recovery phase, was defined as the feet
being pulled forwards with the webbing closed (Fig.·2, frame
5–7). Foot retraction was immediately followed by the power
or thrust phase where the leg was quickly pushed backward
with the webbing extended, until the leg was fully extended
backward (Fig.·2, frame 8–10). Fig.·3 illustrates the stages of
the foot and wing strokes and their timing corresponding with
frames in Fig.·2. This indicates that the thrust phase of foot
strokes corresponded with the transition between the upstroke
and downstroke of the wings, when the angle of attack and drag
across the wings was greatest. Feet were maintained in a
gliding position throughout the remainder of the wing stroke
cycle, even though there was adequate time for another foot
stroke.
THE JOURNAL OF EXPERIMENTAL BIOLOGY
3978 J. P. Heath, H. G. Gilchrist and R. C. Ydenberg
THE JOURNAL OF EXPERIMENTAL BIOLOGY
Fig.·2. Various stages of the wing and foot stoke cycle illustrated from 1/30th of a second video frames of common eiders during descent, from four different angles (rows). Each stroke
cycle illustrated was 0.43·s and so frame numbers from 1 to 13 are used to describe the various stages of the stroke cycle in the text, and correspond with stoke cycle stages indicated
in Fig.·3.
Diving in currents 3979
THE JOURNAL OF EXPERIMENTAL BIOLOGY
Glide
Retract
Kick thrust
Upstroke
Transition
Downstroke
12⬘
1
3
5
7
9
11
13
2⬘
4⬘
6⬘
Frame number (1/30th second)
Fig.·3. Timing of various stages of the wing and foot stroke cycle.
Wing (black circles) and foot (white circles) stokes are divided
into three stages, which are illustrated in Fig.·2 and described in
the text. Thrust from foot propulsion ubiquitously corresponded
with the transition between the upstroke and downstroke of the
wings, when drag is probably greatest because of the large angle of
attack of the wings. Correspondence in timing between power and
recover phases of wing and foot propulsion could therefore be
important in maintaining steady speed, minimizing the cost of drag
during diving.
No. of wing stroke cycles
50
A
40
30
20
10
22
Descent duration (s)
20
Diving behaviour in relation to current velocity
As predicted, descent duration increased non-linearly with
increasing current velocity to a constant depth of 11.3·m
(quadratic regression: R2=0.367, d.f.=155, P<0.0001; Fig.·4A).
Mean (±s.d.) descent duration was 10.47±1.99·s, and so
average vertical descent speed (relative to the bottom) was
1.11±0.17·m·s–1. The number of wing stroke cycles during the
descent also increased non-linearly with current velocity with
an average of 22.01±4.12 strokes per descent (R2=0.395,
d.f.=153, P<0.0001; Fig.·4B). This increase in number of
strokes with current velocity occurred while maintaining a
constant average stroke frequency of 2.28±0.23·Hz across
current velocities (R2=0.00008, d.f.=72, P=0.938; Fig.·5).
Remarkably, effective swim speed (relative to water) showed
only a slight increase across current velocities (R2=0.112,
d.f.=156, P<0.0001; estimated dive angle=10°, Fig.·6) with an
average value of 1.25±0.14·m·s–1 despite a current gradient of
1.0·m·s–1. Regression equations are presented in Table·1.
Discussion
Stroke patterns
Diving birds have often been considered to use either feet
or wings for propulsion. For divers that use lift-based wing
propulsion, deceleration between the upstroke and
downstroke is high, with drag expected to be greatest because
of the high angle of attack of wings (Stettenheim, 1959;
Lovvorn et al., 1999; Lovvorn et al., 2004). For divers
employing drag-based foot propulsion, drag from trailing feet
could be high [(e.g. Pennycuick et al., 1996) for flight] and
deceleration and even negative thrust can occur during the
recovery phase (Lovvorn et al., 1991). Generally, lift-based
wing propulsion is more efficient than drag-based foot
propulsion (Fish, 1993; Fish, 1996; Lovvorn and Liggins,
2002). For each mode of locomotion separately, increased
instantaneous velocity and therefore increased drag costs
during the power phase would be required to counter
deceleration during the recovery phase, in order to maintain
a mean swim speed that keeps work against drag within
B
2.7
18
2.6
16
14
12
10
8
0
0.2
0.4
0.6
0.8
Current velocity (m
1.0
s –1)
Fig.·4. Descent duration (A) and number of wing stroke cycles (B) to
descend to depth (11.3·m) increased non-linearly with increasing
current velocity (m·s–1). Note that each wing stroke cycle also
included a foot stroke.
Stroke frequency (Hz)
Wing stroke
Foot stroke
3980 J. P. Heath, H. G. Gilchrist and R. C. Ydenberg
2.5
2.4
2.3
2.2
2.1
2.0
1.9
0
0.2
0.4
0.6
Current velocity
0.8
1.0
(m s–1)
Fig.·5. Average stroke cycle frequency per dive did not vary with
respect to current velocity.
THE JOURNAL OF EXPERIMENTAL BIOLOGY
Diving in currents 3981
Table·1. Regression equations for descent durations and wing
strokes versus current velocity; vertical descent speed relative
to the bottom and effective swim speed relative to the water as
a function of current
1.5
Speed (m s–1)
1.4
1.3
Descent=8.938+3.192⫻Current+5.616(Current–0.3528)2
Stroke=18.507+8.518⫻Current+5.737(Current–0.3528)2
Speedrel=1.243–0.302⫻Current–0.3428(Current–0.3528)2
Speedeff=1.191+0.175⫻Current
1.2
1.1
1.0
0.9
0.8
0
Effective swim speed
95% CI
Descent speed
0.2
0.4
0.6
Current velocity
0.8
1.0
(m s–1)
Fig.·6. Regression equation of vertical descent speed (relative to the
bottom; solid line) and effective swim speed relative to the fluid
(calculated using vector geometry; see Materials and methods), over
a range of current speeds. The dashed line is regression of effective
swim speed at a dive angle of 10° with 95% confidence intervals. The
shaded area indicates regression equations from sensitivity analysis of
dive angle from 0° (lower edge of shaded area) to 20° (upper edge of
shaded area). Effective swim speed was regulated across currents at a
relatively constant value of 1.24±0.14·m·s–1 while vertical descent
velocity decreased non-linearly. Regression equations are presented in
Table·1.
desirable bounds (Lovvorn, 2001; Lovvorn and Liggins,
2002). Maintaining a more steady velocity through the stroke
cycle can therefore reduce energetic costs of diving (Lovvorn,
2001).
Common eiders diving in an arctic polynya used both wings
and feet to power their descent. Diving eiders exclusively timed
the power phase of foot propulsion with the transition between
the upstroke and downstroke of the wings, when drag is high
across the steep angle of attack of wings (Figs·2 and 3). This
timing could reduce drag costs if it provided a more steady
velocity through both foot and wing propelled stroke cycles.
For example, the use of multiple propulsors by boxfishes
(family Ostraciidae) can allow smooth thrust production and a
steady trajectory during swimming (Hove et al., 2001; Gordon
et al., 2000). Similarly, river otters (Lutra canadensis)
coordinate hindlimb and tail propulsion to maintain a more
constant velocity during undulatory swimming (Fish, 1994).
Integrating synergistic interactions among different
appendages during locomotion will be important for
understanding the energetics of swimming (see Dickinson et
al., 2000). Our observation of co-ordinated timing of foot and
wing stroke patterns suggests an important mechanism by
which diving birds could maintain steady velocity and reduce
costs of diving.
Confirming that the alternating timing of wings and feet
stroke patterns provides the most efficient propulsion strategy
for eiders would require measuring instantaneous velocity
throughout the stroke cycle (using accelerometers or spatially
referenced video) on birds diving with different foot–wing
stroke patterns, ranging from just wings or feet, to all temporal
Descent, descent duration (s).
Stroke, wing strokes (counts).
Current, current velocity (m·s–1).
Speedrel, vertical descent speed relative to the bottom.
Speedeff, effective swim speed relative to the water.
Relevant statistical analyses are presented in the text.
combinations. This will be a difficult empirical challenge and
further insight will probably come from modelling (e.g.
Lovvorn et al., 2004). Eiders kept their feet extended (glide
phase) for a large portion of the wing stroke cycle during which
there was adequate time to complete an additional foot stroke.
This could have allowed foot thrust during the transition
between the downstroke and upstroke of the wings. The angle
of attack of wings was low during this stage, so deceleration
due to drag may not be adequate to necessitate an additional
foot stroke, particularly if drag during retraction of feet
interferes with gliding. Undulation of the neck and body, as we
observed for eiders, could also be an important consideration
in estimating swimming costs based on dead-drag
measurements (Blake, 1983).
Influence of current velocity
At dive depths up to 11.3·m studied in this analysis,
energetic costs to counter buoyancy during diving are
important but decrease with depth (Wilson et al., 1992;
Lovvorn et al., 2004). Drag increases at a slower rate than
buoyancy decreases with depth, however, drag can be one of
the most important factors influencing the energetics of
diving, particularly as buoyancy decreases (Lovvorn, 2001;
Lovvorn et al., 2004). Drag is especially important in relation
to increasing swim and current speed, as drag increases
rapidly with increasing speed (Lovvorn et al., 1991).
Regulation of swim speed within a narrow range that controls
drag costs has been observed over a range of depths for diving
Brünnich’s guillemots [1.6±0.2·m·s–1 over presumably little
current (Lovvorn et al., 2004)] and flight speeds of a variety
of birds (Pennycuick, 1997). Our results indicate that eiders
maintained a constant stroke frequency, and that effective
swim speed (relative to the fluid) only slightly increased as
current velocity increased (Figs·5 and 6). Overall, effective
swim speed was therefore regulated at 1.25±0.14·m·s–1,
which is particularly impressive given that current velocities
ranged from 0.04–0.97·m·s–1 during these observations.
Although a distinct breakpoint of increasing drag relative to
speed is not obvious from tow-tank measurements for frozen
THE JOURNAL OF EXPERIMENTAL BIOLOGY
3982 J. P. Heath, H. G. Gilchrist and R. C. Ydenberg
eiders, the steeply accelerating drag curve certainly suggest
that swim speed is regulated to limit work against rapidly
increasing drag (Lovvorn et al., 2001; Lovvorn et al., 2004;
Watanuki et al., 2005; Dial et al., 1997). The constant stroke
frequency we observed may correspond to an efficient
maximum for eiders.
Regulation of swim speed and stroke frequency by necessity
resulted in a decrease in vertical descent speed relative to the
bottom, and therefore increased descent duration and the
number of stroke cycles required to descend to the bottom.
While tightly regulated, effective swim speed significantly
increased by approximately 0.18·m·s–1 with current velocity.
Overall, this would suggest an important increase in metabolic
rate and oxygen consumption during descent in faster currents
(Hawkins et al., 2000), and the increase in number of wing
strokes probably indicates increasing energetic costs
(Williams et al., 2000; Williams et al., 2004). Therefore, while
common eiders employ a variety of tactics to reduce energy
costs during diving (timing of foot and wing strokes,
regulation of stroke frequency and swim speed), increasing
currents nevertheless increase the time and energy costs of
descent to foraging patches. These costs are expected to have
a strong influence on time allocation over the dive cycles,
particularly on time available to forage at depth (Houston and
Carbone, 1992). During fast currents, common eiders
wintering at Ulutsatuq polynya get out of the water and rest
on the ice edge. Although swim speeds required to descend in
fast currents could be possible, the strong increase in drag and
energetic costs of diving potentially make foraging
unprofitable. Consideration of these factors will be particularly
important in developing our understanding of the energetics of
diving by free ranging birds.
Although diving ecology is the focus of the present
manuscript, this study would not have been possible without
the assistance of local ecological knowledge from the Inuit of
Sanikiluaq. Their assistance helped to locate an ideal study
site and establish the logistical support in the field necessary
to successfully record underwater videos; particularly in
identifying sea ice on which it was safe to walk and set up
research equipment. We literally followed in their footsteps
for this research, for which we are gratefully indebted.
Thanks to numerous people for field work and logistical
support, particularly Karel Allard, Lucassie Arragutainaq,
Rachael Bryant, Lucassie Ippaq, Simeonie Kavik, Elijah
Oqaituk, Myra Robertson, Dwayne Searle and Paul Smith.
Thanks to Dan Esler, Tony Williams, Jim Lovvorn and an
anonymous reviewer for helpful comments on the
manuscript. Support was provided by the Canadian Wildlife
Service, Nunavut Community Development Fund, Northern
Ecosystem Initiative, Wildlife Habitat Canada, World
Wildlife Fund Canada, Polar Continental Shelf, Association
of Canadian Universities for Northern Studies, Nunavut
Wildlife Management Board, ArcticNet National Centre of
Excellence Theme 3.6, University of Manitoba, Northern
Scientific Training Program, the Centre for Wildlife Ecology
at Simon Fraser University and an NSERC PGS-B
scholarship to J.P.H.
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